The invention relates to a dual-catalyst system for, and a catalytic preparation of, bimodal molecular weight distribution polymers and copolymers of ethylene, produced in one reactor. A make-up catalyst (catalyst one) and a bimetallic catalyst (catalyst two) are used to control the proportion of different weight fractions in broad or bimodal molecular weight distribution olefin resin product. The bimetallic catalyst produces broad or bimodal molecular weight distribution polyolefin resin, in which there are at least two components; one of the two components has a higher molecular weight (referred herein as HMW) than another of the two components, which is referred herein as LMW, low molecular weight (component). The bimetallic catalyst produces broad or bimodal molecular weight distribution polyolefin resin whose composition depends on the ratio of the concentration of the two metals of the catalyst producing the HMW/LMW components.
The make-up catalyst, consisting of a single metal component, is added in proportion necessary to make-up the deficiencies in the amount of the HMW/LMW component produced by the bimetallic catalyst. The type of make-up catalyst added depends on whether increase of the HMW or LMW component produced by the bimetallic catalyst is sought. A make-up catalyst of at least one metallic component of the bimetallic catalyst is used to control the weight fraction of the HMW component in the broad or bimodal molecular weight distribution olefin resin product.
In U.S. Pat. No. 5,032,562, a bimetallic catalyst system is described that produces broad or bimodal molecular weight distribution polyethylene resins in a single reactor. The LMW component is produced by the Zr site while the HMW component is produced by the Ti site.
The final product is essentially a gel-free intimate mixture of these two components because of the close proximity of the two metal sites.
In that product, the weight fraction of the HMW/LMW components in the final product is fixed by the relative concentration of the two metal species in the bimetallic catalyst. It is often desirable to change the weight fraction of either the HMW or LMW components produced by bimetallic catalyst in the single reactor. Reformulation of the bimetallic catalyst offers one mode of operation, to change the relative weight fraction of one of the two components produced by such a catalyst. In accordance with the invention, time consuming reformulation procedure(s) are eliminated by employing a make-up stream of the pure component such that the desired composition of the HMW/LMW component is achieved. In the absence of the bimetallic catalyst, (i.e.) cofeeding one catalyst containing only the titanium (HMW) metal and a second catalyst containing only the zirconium (LMW) metal, the polymer will essentially be a physical mixture of the two components and severe restrictions will have to be placed on the allowable weight fraction of the two components for producing a gel free product with acceptable level of fines (less than 6 percent of the polymer) in the reactor.
The invention relates to a catalyst and a catalytic preparation of bimodal molecular weight distribution polymers and copolymers of ethylene, in one reactor. A make-up catalyst and a bimetallic catalyst are used simultaneously to control the proportion of different weight fractions in broad or bimodal molecular weight distribution polyolefin resin product. The invention relates to a catalyst system comprising (a) a bimetallic catalyst and (b) a make-up catalyst. The catalyst system comprises greater than 50 weight percent of the bimetallic catalyst. It is important to have the bimetallic catalyst as the primary catalyst supply to minimize gel formation. The bimetallic catalyst contains two transition metals or two different transition metal compounds, each having different hydrogen characteristics. Because of the different hydrogen response of each of the two sources of transition metal in the bimetallic catalyst, each will produce a different molecular weight component under identical olefin polymerization conditions. In preferred embodiments below, the LMW component is produced by the Zr site while the HMW component is produced by the Ti site.
The bimetallic catalyst produces broad or bimodal molecular weight distribution polyolefin resin whose composition depends on the ratio of the concentration of the two transition metal catalyst components producing the HMW/LMW components. Thus the product of olefin polymerization in the presence of such a catalyst will comprise at least two fractions each of different molecular weight, and one having a relatively high molecular weight (hereinafter HMW) relative to a second fraction of relatively lower molecular weight (LMW).
In that product, the weight fraction of the HMW/LMW components in the final product is fixed by the relative concentration of the two metal species in the bimetallic catalyst. The make-up catalyst consists of a single metal component of the two components constituting the bimetallic catalyst; it is added in proportion necessary to increase either the amount of the HMW or the LMW component. The identity of the transition metal in the make-up catalyst added depends on whether increase of the HMW or of the LMW component produced by the bimetallic catalyst is sought. The make-up catalyst (b) is added, for example, as a make-up stream of the pure catalyst components such that the desired composition of the HMW/LMW components is achieved. In preferred embodiments below, a pure Ti catalyst can be co-fed if the HMW component weight fraction needs to be increased. In preferred embodiments below, a zirconium based (zirconocene) catalyst is co-fed to increase the LMW component weight fraction.